This disclosure relates to a method and apparatus that in general balances the rate of ink-jet printhead nozzle firing with the density of ink deposited per unit area, and in particular resolves differences between the format of the print data and the print dot grid.
It is common for print data to have a format that differs from the print dot grid used to print that data. For example, an 8-bit, 16-bit or higher color resolution RGB (red, green, blue) color data file for display on a CRT (cathode ray tube) or similar display may be mapped to a 1- or 2-bit CMYK (cyan, magenta, yellow, black) color data file for operation of a printer. In this case, the RGB data file has much greater color resolution, but has much lower dot or pixel resolution. Halftoning algorithms attempt to avoid data loss in the translation by finding a means to associate the color depth of the RGB file with the greater resolution of the CMYK file. After halftoning, some mapping or translation of the resulting CMYK file may be necessary, depending on the printer""s resolution.
For example, FIG. 1, Prior Art, shows an exemplary post-halftoning process 100, wherein each pixel within an input print data file is mapped to a whole number of pixels within an output dot grid data file. A representation of an input print data file 102, may be the output of a halftoning process, and contains pixels having a resolution of 600 by 600 dots per inch (dpi). A mapping process 104 maps the input print data file 102 into an output dot grid data file 106 having a 1200 by 600 dpi pixel resolution.
Each pixel 110 within the input print data file may have a numeric value 108, representing the color depth. For example, the color depth may be 2-bit; that is, each pixel of color data is associated with a number from 0 to 3. In a typical prior art application, the color depth may be restricted to include only numbers from 0 to 2. Thus, during the mapping process, each pixel 110 of the print data, having a value from 0 to 2, is associated with a pair of pixels 112 within the output dot grid data file 106. The mapping may be performed by entering either zero or a one in data locations representing the pixels within the output dot grid data file. Three exemplary mappings 114, 116, 118 show how input print data 102 having a value of 2, 1 or 0, respectively, can be mapped to an output dot grid data file 106. In this manner, the prior art is able to map a print data with lower dot resolution and greater color bit depth into dot grid data with higher resolution and less bit depth.
However, as result of this mapping process, data associated with each pixel 110 in the input print data file 102 is mapped to two, three or other whole number multiple of pixels 112 in the output dot grid data file. Accordingly, the printers of the prior art tend to have resolutions such as 300 by 300 dpi, or 600 by 300 dpi, or 1200 by 600 dpi. These increasingly large steps by which resolution is increased can result in many design and output problems.
Generally, a greater number of dots per inch in the output results in a greater ink saturation, and better print output. In one example, a 600 by 600 dpi resolution dot grid may result in four 18 ng (nano gram) dots of black ink (72 ng total) within a {fraction (1/300)} by {fraction (1/300)} square inch unit area. This may be inadequate for optimal coverage. By increasing to 1200 by 600 dpi, 144 ng of ink per inch is deposited. However, using a 45 inch per second rate of printhead movement along the carriage, the 1200 by 600 dpi rate may require a pen speed (rate of printhead nozzle firings) of 54 KHz, which may be beyond the capability of the hardware. This may require slowing of the rate of printhead movement, and therefore the rate of overall printing, or other equally unattractive option.
Thus, it can be seen that in the prior art, the options available to the printer designer may include too little ink saturation, expensive hardware upgrades to support faster pen speed or slowing the overall rate of print output. Accordingly, there is a need to provide better choices for the printer design engineer.
A system and method of printing allows mapping between an input print data file and an output dot grid data file, wherein the resolution of the dot grid is not an integer multiple of the print data. A method includes forming an input print data group of pixels within the print data file and forming an output dot grid group of a different number of pixels within the dot grid data file. The pixels within the input group are associated pixels within the output group by a mapping function. By varying the number of pixels within the two groups, the printhead nozzle firing frequency and the rate of ink deposition per unit area of media may be controlled. Accordingly, print output quality may be maximized given the constraints of the hardware available.